1. Field of the Invention
The present invention relates to a toner used for electrophotography, electrostatic recording, magnetic recording, and toner jet recording, and to a process for producing the toner.
2. Related Background Art
Conventional electrophotography comprises forming an electrostatic image on a photoreceptor by various means, then developing the electrostatic image with a toner to form a toner image on the photoreceptor, transferring the toner image onto a transfer material such as paper if required, and then fixing the toner image onto the transfer material by fixing means such as heat, pressure, heat with pressure, or solvent vapor to obtain an image (see e.g. Society of Electrophotography of Japan (ed.), “Fundamentals and Applications of Electrophotographic Technology” (Denshishashin Gijutsu no Oyo to Kiso), Colona Publishing Co., Ltd., Jun. 15, 1988, pp. 46-79).
Various conventional methods for developing with a toner or fixing a toner image have been proposed and employed for respective image-forming processes in a suitable manner. Conventionally, toners used for these purposes have been generally produced by melt mixing a thermoplastic resin with a coloring agent made of a dye and/or a pigment to produce a resin composition with a coloring agent uniformly dispersed, and providing the coloring agent-dispersed resin composition with a desired particle size by a pulverizer or classifier.
This process for producing these toners can produce a quite excellent toner, but have certain limitations. For example, it is necessary that the coloring agent-dispersed resin composition be adequately fragile and can be pulverized by an economically feasible production apparatus. However, when the coloring agent-dispersed resin composition is made fragile, particles formed by actually pulverizing the composition at a high speed tend to have particle sizes within a wide range and, in particular, may comprise relatively large particles, disadvantageously.
Moreover, such a highly fragile material tends to be further pulverized or powdered when used as a toner for development. In this process, it is difficult to uniformly disperse solid microparticles such as a coloring agent into a resin in a good manner. This process may cause increased fogging, a reduced image density, and decreased color mixing or transparence of the toner, depending on the degree of dispersion. In addition, the coloring agent may be exposed on the broken-out section of the toner to cause a change in development characteristics of the toner.
On the other hand, in order to overcome these problems of a toner produced by pulverization, a process for producing a toner by suspension polymerization has been proposed. Suspension polymerization comprises uniformly dissolving or dispersing a polymerizable monomer, a coloring agent, a polymerization initiator and, if required, a crosslinking agent, a charge control agent and other additives to prepare a polymerizable monomer composition, then dispersing the polymerizable monomer composition in an aqueous dispersion medium containing a dispersion stabilizer with a suitable stirrer, and polymerizing the polymerizable monomer to obtain toner particles with a desired particle size (see e.g. Japanese Patent Publication No. 36-10231, Japanese Patent Publication No. 42-10799, and Japanese Patent Publication No. 51-14895).
This process does not comprise a pulverization step, and thus can use a soft material for toner particles, the material not necessarily fragile, does not allow the coloring agent to be exposed on the surface of the toner particles, and provides the particles with uniform triboelectric charging properties. The process can also omit a classification step, and thus exhibits significant cost reduction effects such as energy savings, a reduced production time and an improved step yield.
Methods for fixing a toner image such as heat pressing by a heat roller (hereinafter referred to as heat roller fixing) and heat fixing while causing a heating body to adhere to a sheet to be fixed via a fixing film (hereinafter referred to as film fixing) have been developed.
Heat roller fixing or film fixing comprises bringing the surface of a heat roller or fixing film into contact with a toner image on a sheet to be fixed, under pressure by a pressing member in contact with the roller or film, to cause the roller or film to pass the sheet, thereby fixing the toner image. This fixation method allows the surface of a heat roller or fixing film to be brought into contact with a toner image of a sheet to be fixed, and therefore exhibits extremely high thermal efficiency in melting a toner image onto the sheet and enables rapid and good fixing.
Electrophotographic apparatuses in recent years have been demanded variously to provide high image quality, to be downsized and lightened, to be produced at a high speed with high productivity, to save energy, to be highly reliable, to be inexpensive, and to be maintenance-free. In particular, important technical objectives for a fixing step are to develop systems and materials that can achieve further high-speed production, energy savings, and high reliability. However, in order to achieve these objectives with heat roller fixing or film fixing, it is essential to improve fixing properties of a toner as a material considerably, and it is necessary to improve properties that can make a toner fixed onto a sheet to be fixed sufficiently at a lower temperature (hereinafter referred to as low-temperature fixing properties) and to improve properties that can prevent an offset as a phenomenon in which the toner contamination attached onto the surface of a heat roller or film contaminates the next sheet to be fixed (hereinafter referred to as offset resistance), in particular.
Toners used for fixing with heat and pressure, which contain a wax with high affinity for a binder resin, exhibit good offset resistance and low-temperature fixing properties under specific fixing conditions (see e.g. Japanese Patent Application Laid-Open No. H5-50367 and Japanese Patent Application Laid-Open No. 2001-318484). Toners containing two or more waxes with different affinities for a binder resin can exhibit good low-temperature fixing properties and improved offset resistance under specific fixing conditions (see e.g. Japanese Patent Application Laid-Open No. 60-252361, Japanese Patent Application Laid-Open No. H8-50367, Japanese Patent Application Laid-Open No. 2001-324834, and Japanese Patent Application Laid-Open No. 2002-72534). However, since these toners have a lower glass transition temperature as the wax is compatible with a binder resin, the toners tend to have impaired storage stability, flowability, and charging properties, and easily cause a remarkable density reduction and image defects particularly when continuously printed. Therefore, a toner with satisfactory storage stability and development stability and enhanced low-temperature fixing properties has been desired.
An object of the present invention is to provide a toner that can solve the above-described problems.